The present invention relates to a method and apparatus for forming a multiple-element thin film based on ion beam sputtering.
A variety of techniques have been proposed in connection with the method and apparatus for forming multiple-element thin films. They base the principle of evaporation on the utilization of the usual sputtering phenomenon, evaporation by electron beam heating, sputtering with a single ion beam source or neutral beam source and one or more targets or with a plurality of targets and ion beam sources, for example.
In any film forming method and apparatus, the composition control is determined from the relation between the quantity and condition of film formation on a trial-and-error basis, and therefore if the surface condition or the like of the target changes or if the atmosphere of film formation changes when the experiment is put into practice, the sputtering rate and the rate of deposition on the substrate will vary, resulting in a deviated film composition. This problem is attributable to the absence of in-situ monitoring, i.e., on-the-spot observation, of the composition during the film formation, and the absence of a facility for conditioning the film formation based on the result of monitoring. Techniques related to this affair are described in JP-A-63-53265 and JP-A-63-241822, for example.
Among the above-mentioned conventional techniques, evaporation of usual D.C. sputtering phenomenon involves such a problem that an insulator target made of oxide or the like cannot be sputtered. Application of high-frequency sputtering which can deal with insulator targets encounters the difficulty in controlling the sputtering speed of film formation independently because of the electrical linkage of voltages applied to the target electrodes, leaving a problem of inaccurate composition control. The atmosphere of film formation is variable only in the domain where plasma for sputtering the target exists stably, and it is limited in the adjustment range of composition. On this account, in forming a compound thin film including liquid or gaseous elements at the room temperature, these elements volatiles into the atmosphere, and therefore the range of composition control for these elements is limited for methods based on reactive sputtering. The above-mentioned sputtering techniques necessitate the creation of plasma on the target, which causes high-energy ions or electrons to be injected into the created film, resulting possibly in a decayed film. The emergence of discharging or plasma luminescence in the proximity of the substrate disables the in-situ monitoring for the composition of sputtered particles through the method which does not render electrical disturbance to the interior of the film formation chamber, and therefore indirect measurement of the particle composition based on the measurement of the film thickness is required.
In the case of evaporation by electron beam heating or the like, the range of evaporation control for each constituent is small and the quantity of evaporation varies slowly in response to the heating power, leaving a problem in inaccurate composition control even with the provision of an in-situ composition monitor. Reactive deposition of oxide is prone to encounter the oxidation inside the crucible and instability of the evaporation area in the crucible, leaving a problem in composition control.
An example of film formation which is high in controllability of composition and satisfactory in crystal formation is the moleculer beam epitaxial method. However, it encounters the difficulty in gas introduction and the like due to the restriction of film material by the fusion point and vapor pressure and the environment of high vacuum, leaving a problem of difficult thin film formation for oxide and nitride.